This invention relates to an improved heater bar that can be used for either a heater in a laser printer fuser or a heater in a thermal overcoat or laminating mechanism. The improved heater bar utilizes a positive temperature coefficient (PTC) thermistor element to produce the heat output.
It is known, in printing devices, to measure the temperature of the print head. Exemplary of such prior art are U.S. Pat. No. 4,996,567 (""567) to S. Watarai et al., entitled xe2x80x9cMethod of Controlling Fuser Unit of Image Forming Apparatus,xe2x80x9d U.S. Pat. No. 5,481,089 (""089) to H. Furuta, entitled xe2x80x9cHeater Control Device for Image Forming Apparatus,xe2x80x9d U.S. Patent No. 5,844,694 (""694) to S. Miura, entitled xe2x80x9cFacsimile Apparatus,xe2x80x9d U.S. Pat. No. 5,986,241 (""241) to N. Funahashi, entitled xe2x80x9cHeating Control System for Heater Provided in Laser Printer,xe2x80x9d and U.S. Pat. No. 6,172,699 (""699) to T. Ohashi, entitled xe2x80x9cThermal Printing System Having Function for Preventing Over Heating of Thermal Head.xe2x80x9d All of the above references are concerned with measuring the temperature of a device with a sensor and then controlling the temperature of the device with some type of feedback control circuit. Consequently, a more advantageous system, then, would be presented if the temperature sensor and feedback control circuit were eliminated.
It is also known to employ positive temperature coefficient (PTC) thermistors. These devices have a resistance-temperature characteristic that exhibits a very small negative temperature coefficient until the device reaches a critical temperature, which is referred to as its xe2x80x9cCuriexe2x80x9d, switch or transition temperature. As this critical temperature is approached, the devices begin to exhibit a rising, positive temperature coefficient of resistance as well as a large increase in resistance. This resistance change can be as much as several orders of magnitude within a temperature span of a few degrees.
It is further known to use PTC thermistors in heater bars. The Bahle Corp. of Puchon, Korea employs PTC thermistors in thermal binding machines and laminating machines. In these machines, the PTC thermistor is located in the heating element. However, due to the nature of the heating element, the heating element has a lot of thermal mass and would take some time to heat up. Also, there is no good thermal connection between the PTC thermistor and the frame. In fact, all the elements used to hold the PTC thermistor in place act as an insulator between the PTC thermistor and the frame. Consequently, a more advantageous system, then, would be presented if the amount of thermal mass and insulation were substantially reduced.
It is apparent from the above that there exists a need in the art for a heater bar which is lightweight through simplicity of parts and uniqueness structure, and which at least equals the heating characteristics of the known heaters, particularly those which employ the highly advantageous PTC thermistor, but which at same time avoids the use of temperature sensors and feedback control circuits. It is a purpose of this invention to fulfill this and other needs in the art in a manner more apparent to the skilled artisan once given the following disclosure.
Generally speaking, this invention fulfills these needs by providing a heater bar assembly, comprising a non-heat conductive frame means, a frame stabilizer means operatively connected to one side of the frame means, a positive temperature coefficient thermistor means located substantially within another side of the frame means, and an electrical conductor means located substantially adjacent to the thermistor means and the frame means.
In certain preferred embodiments, the frame means is constructed of any suitable polymeric material that is capable of withstanding temperatures of at least 150 degrees C. Also, the frame stabilizer means is constructed of any suitable material that is capable of keeping the frame from distorting. Finally, the electrical conductor means includes metallic, electrical conductors located on each side of the thermistor to provide electricity to the thermistor and a low thermal conducting material that is used to hold the metallic conductors and the thermistor in place.
In another further preferred embodiment, the heater of the present invention is capable of providing a uniform temperature along the length of the heater even when using narrow media with a wide heater.
The preferred heater bar, according to this invention, offers the following advantages: lightness in weight; ease of assembly and repair; elimination of temperature controls; temperature uniformity; reduced chance of over heating; excellent economy; increased reliability; good stability; good durability; and ease of power output adaptation. In fact, in many of the preferred embodiments, these factors of lightness in weight, ease of assembly and repair, elimination of temperature controls, temperature uniformity, reduced chance of over heating, reliability, economy, and power output adaptation are optimized to an extent that is considerably higher than heretofore achieved in prior, known heater bars.
The above and other features of the present invention, which will become more apparent as a description proceeds, are best understood by considering the following detailed description in conjunction with the accompanying drawings, wherein like characters represent like parts throughout the several views and in which: